Suspended solids (SS/TSS) determination is one of the most fundamental indicators in water quality analysis, reflecting the concentration of undissolved particulate matter retained by a standard filter. This technical article explains the measurement principles, applicable standards such as ISO 11923 and ASTM D5907, sampling requirements, testing procedures, and typical industrial applications of suspended solids analysis.
Technical Overview
Suspended solids refer to the particulate matter in a water sample that is retained on a standard filter membrane (typically with a nominal pore size of 0.45 µm) and dried to a constant weight at a specified temperature. The result is expressed as total suspended solids (TSS), also known as non-filterable residue, in units of milligrams per liter (mg/L). Suspended solids typically include silt, clay, organic detritus, plankton, microorganisms, and insoluble industrial particulates.
The TSS value is a core parameter in water quality assessment because elevated suspended solids can reduce light penetration, smother aquatic habitats, transport adsorbed pollutants such as heavy metals and pesticides, and interfere with disinfection and membrane processes in treatment plants. Regulatory discharge standards in most jurisdictions specify explicit limits for suspended solids, making accurate and reproducible measurement essential for compliance verification.
Commonly adopted reference standards for suspended solids determination include:
- ISO 11923 — Water quality — Determination of suspended solids by filtration through glass-fibre filters
- ASTM D5907 — Standard Test Methods for Filterable and Nonfilterable Matter in Water
- EN 872 — Water quality — Determination of suspended solids — Method by filtration through glass fibre filters
- GB 11901 — Water quality — Determination of suspended substance — Gravimetric method
- USEPA Method 160.2 — Residue, Non-Filterable (Gravimetric, Dried at 103–105 °C)
Testing Samples
Environmental Water Samples
Natural water bodies such as rivers, lakes, reservoirs, estuaries, coastal seawater, and groundwater are routinely analyzed for suspended solids. These samples characteristically exhibit wide concentration ranges, from less than 5 mg/L in deep groundwater to several thousand mg/L in turbid rivers during flood events. Representative sampling across depth and flow conditions is critical, since suspended particles are subject to settling and stratification.
Municipal and Industrial Wastewater
Influent, intermediate, and effluent streams from municipal wastewater treatment plants, as well as wastewater from industries such as pulp and paper, mining, food processing, petrochemicals, and textiles, are tested for TSS. Effluent samples are compared against permitted discharge limits, while influent and mixed-liquor samples support process control in primary clarification, activated sludge, and secondary settling operations.
Treated, Process, and Recirculating Water
Drinking water treatment streams, membrane system feed and permeate, boiler feedwater, cooling tower circulation water, and reclaimed water are monitored for low-level suspended solids. In these matrices, TSS is often evaluated alongside turbidity, since concentrations may approach the practical detection limit of the gravimetric method.
Testing Items
A comprehensive suspended solids evaluation typically covers the following measurable items:
- Total suspended solids (TSS) — the mass of filterable residue dried at 103–105 °C, expressed in mg/L
- Volatile suspended solids (VSS) — the organic fraction lost on ignition at approximately 550 °C, used to estimate biomass content in biological treatment systems
- Fixed suspended solids (FSS) — the inorganic, non-combustible residue remaining after ignition
- Total dissolved solids (TDS) or filterable residue — the complementary fraction passing through the filter, often determined simultaneously
- Settleable solids — the volume of particulate matter that settles under quiescent conditions, typically measured with an Imhoff cone
- Particle size distribution and turbidity correlation — supplementary analyses used to characterize suspension behavior
Testing Methods
Gravimetric Filtration Method (Reference Method)
The internationally recognized reference procedure is based on filtration followed by gravimetric determination. A pre-cleaned glass fiber filter or 0.45 µm membrane filter is conditioned, dried to constant weight, and accurately weighed. A measured volume of well-mixed water sample is drawn through the filter under vacuum. The filter with retained solids is then dried in an oven at 103–105 °C to constant weight, cooled in a desiccator, and reweighed. The suspended solids concentration is calculated from the mass gain divided by the filtered sample volume.
Key operational considerations include:
- Homogenizing the sample thoroughly before aliquoting, as particles settle rapidly
- Selecting an appropriate filtration volume so that the retained residue mass is sufficient for accurate weighing (typically at least 1–5 mg, ideally 10–100 mg)
- Verifying constant weight, defined as a mass change of no more than 0.4 mg (or 4% of residue mass) between successive drying cycles
- Checking filter blanks and applying corrections where necessary
The practical reporting limit of the gravimetric method is generally around 1–2 mg/L for standard sample volumes; higher sensitivity can be achieved by filtering larger volumes.
Volatile and Fixed Solids Determination
To distinguish organic from inorganic particulate matter, the dried filter residue is ignited in a muffle furnace at 550 ± 50 °C for a defined period. The mass lost during ignition represents volatile suspended solids, while the remaining ash corresponds to fixed suspended solids. This differentiation is particularly valuable in activated sludge process control, where the VSS/TSS ratio serves as an estimator of the active biomass fraction (mixed liquor volatile suspended solids, MLVSS).
Alternative, Rapid, and Online Methods
For process monitoring where laboratory gravimetry is too slow, several instrumental alternatives are used. Turbidity measurement (nephelometry, e.g., ISO 7027, EPA Method 180.1) provides a rapid optical surrogate that can be correlated to TSS for a specific water matrix. Online suspended solids analyzers based on infrared or laser scattering, absorbance, or ultrasonic attenuation enable continuous real-time monitoring at treatment plants. Laser diffraction instruments extend characterization to particle size distribution. However, because optical correlations are matrix-dependent, gravimetric determination remains the definitive arbitration and calibration reference.
Quality Assurance and Quality Control
Reliable results depend on rigorous QA/QC practices, including duplicate determinations (relative deviation typically required to be within 10–20% depending on concentration level), filter and reagent blanks, balance calibration with certified reference weights, oven and furnace temperature verification, and control charts for ongoing performance tracking. Sample preservation at 4 °C and analysis within recommended holding times — ideally within 24 hours and no later than 7 days — prevents microbial alteration of the particulate fraction.
Applications
Suspended solids determination supports a broad spectrum of regulatory, operational, and scientific purposes:
- Environmental compliance — verifying that effluent discharged to receiving waters meets statutory TSS limits set by national and local discharge standards
- Wastewater treatment process control — monitoring primary clarifier efficiency, activated sludge concentration (MLSS/MLVSS), and secondary clarifier performance
- Drinking water and membrane treatment — assessing clarification, coagulation-flocculation, filtration, and ultrafiltration/microfiltration performance and protecting downstream membranes and disinfection
- Construction and dredging management — controlling turbidity and sediment release from earthworks, tunneling, and dredging operations
- Ecological and hydrological research — sediment transport studies, reservoir siltation assessment, and habitat quality evaluation
- Industrial water systems — managing cooling water fouling potential, boiler water quality, and produced water characterization in oil and gas operations
- Product and reclaimed water certification — demonstrating conformance of recycled water and process water to specification
Accredited third-party laboratories routinely perform suspended solids testing under ISO/IEC 17025 management systems, providing legally defensible data for regulatory reporting, environmental impact assessment, litigation support, and contract verification.
FAQ
What is the difference between suspended solids (TSS) and turbidity?
TSS is a direct gravimetric measurement of the mass concentration of particulate matter retained on a filter, expressed in mg/L. Turbidity is an optical measurement of light-scattering intensity, expressed in NTU or FNU. The two parameters are correlated but not interchangeable; the relationship varies with particle size, shape, color, and composition. TSS is preferred for regulatory mass-based limits, while turbidity is preferred for continuous, real-time monitoring.
Why is a 0.45 µm filter used as the operational boundary?
The 0.45 µm pore size has been adopted by convention in international standards as the practical dividing line between "dissolved" and "suspended" fractions. It is not a sharp physical boundary — some colloidal particles smaller than 0.45 µm may be retained, while a few larger particles may pass — but it provides consistency and comparability across laboratories worldwide.
What is the detection limit and typical uncertainty of the method?
With standard filtration volumes (100–500 mL) and analytical balances of 0.1 mg readability, the practical reporting limit is approximately 1–2 mg/L. Larger sample volumes can extend sensitivity. Typical relative uncertainty is in the range of 10–20% for moderate concentrations, with precision improving as the retained residue mass increases. Volatile solids results become unreliable when the TSS is below about 10 mg/L.
How should water samples for suspended solids analysis be collected and stored?
Samples should be collected in clean glass or plastic containers, homogenized gently before subsampling, preserved at 4 °C, and analyzed as soon as practicable — ideally within 24 hours and no later than 7 days. Samples must never be filtered in the field unless a validated field filtration protocol is applied, and freezing should be avoided because it alters particle structure.
How long does a suspended solids test take?
A single determination typically requires several hours to one working day, primarily due to the drying-to-constant-weight steps. Volatile solids determination adds an ignition cycle and may extend the turnaround to 1–2 working days. Routine reporting by accredited third-party laboratories is commonly within 3–7 working days, depending on the scope of the testing program.